The present invention relates to a semiconductor device and a method of manufacturing the same such that the leakage current can be restrained and the gate insulation film can be made thinner.
Enhancement of the degree of integration and the operating speed of transistors has been realized by miniaturizing the transistors based on the scaling rule. Thinning of the gate insulation film has progressed, and, for example, in transistors with a gate length of 0.1 μm or less, it may be necessary to reduce the thickness of the gate insulation film to or below 2 nm. Ordinarily, polycrystalline silicon (hereinafter referred to as poly-Si) has been used as a gate electrode material. The reason lies in that when poly-Si is thus used, the interface between the gate electrode and the gate insulation film therebeneath is stable, and it is easy to introduce an impurity into poly-Si by techniques such as implantation, diffusion, etc., so that it is possible, by selecting the element and concentration of the impurity, to provide each of N-channel MOS field effect transistors (hereinafter referred to as NMOSFET) and P-channel MOS field effect transistors (hereinafter referred to as PMOSFET) with a gate electrode having an optimum work function and to obtain an optimum threshold value.
However, attendant on the progress of miniaturization of transistors, the problem of depletion of the gate electrode has come to be conspicuous. The depletion of the gate electrode is a phenomenon which is difficult to restrain, since poly-Si is a semiconductor. To cope with this problem, it has been widely reported that the depletion of the gate electrode can be restrained by forming a metallic film, in place of poly-Si film, directly on the gate insulation film, and attention has been paid to the development of a metal gate.
However, in the case where the metal gate is formed of a single metal, the work function of the gate electrode is the same for both the NMOSFET and the PMOSFET; therefore, unlike the case of the conventional poly-Si gate, it is difficult to control the work functions of the gate electrodes of the NMOSFET and the PMOSFET, and it may be impossible to obtain an appropriate threshold value. To overcome this problem, there has been proposed a dual-metal gate, in which metallic materials are so selected that the metal gate electrode of the NMOSFET has a work function similar to that of the N-type poly-Si whereas the metal gate electrode of the PMOSFET has a work function similar to that of the P-type poly-Si (see, for example, Chang Seo Park, Byung Jin Cho, Dim-Lee Kwong “Thermally Stable Fully Silicided Hf-Silicide Metal-Gate Electrode,” IEEE ELECTRON DEVICE LETTERS, Vol 25, No. 6, June 2004).
For obtaining a threshold suited to an NMOSFET, a metallic material having a work function around 4.0 eV is suitable. Though hafnium (Hf), zirconium (Zr) and the like have work functions suitable for NMOSFET, they are high in reactivity and, hence, would cause reduction of the underlying gate insulation film (see, for example, Y. Akasaka et al. “Material Selection for the Metal Gate/High-K Transistors,” Ext. Abst. SSDM 2004, p. 196). Besides, in this case, reactivity between the gate insulation film and the gate electrode is so high that the gate insulation film would become thinner, which may increase the leakage current.
The leakage characteristics in the case where hafnium (Hf) is used for the gate electrode and silicon oxide (SiO2) is used for the gate insulation film were evaluated. The results will be described referring to FIG. 9 which is a diagram showing the relationship between gate voltage and leakage current. As shown in FIG. 9, it was found that a rise in gate electrode increases leakage current. This indicates that hafnium (Hf), which is high in reactivity with silicon oxide, breaks the gate insulation film formed of silicon oxide (SiO2), thereby increasing the leakage current.